I. Field of the Invention
This invention relates to a material for optical recording utilizing photochemical hole burning.
II. Description of the Related Art
Materials for optical recording utilizing the photochemical hole burning (PHB) phenomenon are known. These optical recording materials comprise a matrix component called a host component and a photoreactive compound called a guest component which undergoes photochemical reaction upon absorbing photoenergy. By irradiating the recording material with light having high monochromaticity at an extremely low temperature (about the temperature of liquid helium), only the guest component molecules which absorb the light are selectively excited, so that a sharp hole is formed in the absorption spectrum. Thus, based on the presence or absence of such a hole, optical recording can be accomplished under photon mode. Further, by sequentially recording information changing the wavelength of the irradiating light, multiple recording can be made on the same place of the same recording material. Therefore, a high recording density of about as much as 1000 times that attained by the conventional optical digital recording such as compact disk (CD) or laser disk (LD) may potentially be attained.
To increase the wavelength multiplicity for increasing the recording density, it is preferable to employ an amorphous medium as the host component in order to give diversity of the state of dispersion of the guest component. Therefore, organic polymers and silicate glass have been employed as the host component. The conventional PHB materials include a material containing tetraphenylporphine as the guest component and polymethyl methacrylate as the host component (Japanese Journal of Optics, 14(4)263-269, 1985), a material containing quinizarin as the guest component and silicate glass as the host component (Journal of Applied Physics, 58(9)3559-3565, 1985), a material containing free base porphine as the guest component and an aliphatic hydrocarbon as the host component, a material containing Cresyl Violet as the guest component and polyvinyl alcohol (PVA) as the host component, and a material containing phthalocyanine as the guest component and an aliphatic hydrocarbon as the host component.
However, the PHB phenomenon with these conventional PHB materials is unstable at a temperature higher than the temperature of liquid helium, so that recording, storing and reading information are unreliable at such a high temperature. This is due to the fact that an irreversible change in the structure of the PHB material occurs, so that the microscopic structure in the vicinity of the guest component is changed. To overcome this problem of thermal unstableness, a PHB material containing an ionic porphine derivative as the guest component and an organic polymer as the host component was provided (U.S. Pat. No. 5,051,337, Japanese Laid-open Patent Application (Kokai) No. 2-45). Especially, with the PHB material employing polyvinyl alcohol as the host component, since the structural change of the material is inhibited by the hydrogen bonds formed among the host component molecules, holes can be formed at a temperature as high as the temperature of liquid nitrogen.
However, such PHB materials containing a porphine derivative as the guest and a polymer as the host have a drawback in that the hole width is large, so that the wavelength multiplicity which is the most advantageous feature of the PHB materials is reduced.
As for the temperature characteristics of the PHB materials, Thijssen et al reported the formation of holes at a temperature lower than 30K (Chemical Physics Letters, 92(1), 7-12, 1982), Tani et al reported storage of holes at a temperature lower than 60K (Journal of Applied Physics, 58(9), 3559-3565, 1985), and Sakoda et al reported the PHB phenomenon at temperatures not lower than the temperature of liquid nitrogen of the above-mentioned material containing a porphine derivative having an ionic substituent as the guest and PVA as the host (Japanese Journal of Applied Physics, 28 Supplement 28-3, 229-233, 1989). On the other hand, Friedrich et al al reported the PHB of the material containing quinizarin as the guest dispersed in deuterated ethanol. However, they found no significant differences between the PHB phenomena when the deuterated ethanol and ordinary ethanol are used, respectively.